Lactide is polymerizable to high molecular weight polylactic acids which are of great interest for their hydrolytic and biodegradable properties. Such polylactic acids have long been of interest for biomedical uses as sutures and staples. More recently, they have become of interest for the manufacture of articles for non-biomedical uses in which the article would be hydrolytically degradable in the environment to one or more environmentally acceptable waste materials. For most, if not all such uses, it is preferred that the degradable polymer be made from L-lactide. However, lactide made by existing process technology can be very expensive versus alternative materials for non-medical uses because yields can be low and unwanted by-products tend to form during the reaction.
Lactide is most conveniently prepared by polymerizing lactic acid to a relatively low molecular weight oligomeric polylactic acid, which is then heated, generally in the presence of a catalyst to depolymerize it to lactide, i.e., a cyclic dimer which contains two [OCH(CH.sub.3)CO] units, which is then recovered as a component of a vapor product stream. Processing techniques of this type can be seen, for example, in the following references: Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,163 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis, U.S. Pat. No. 4,727,163 (1988); and Muller, Ger. Pat. Applications 3632103 and 3708915 (1988). The prior art processes, however, can entail several drawbacks:
In the depolymerization of a low molecular weight oligomer formed from lactic acid, water and acid impurities can also be formed so that the depolymerization product generally contains these hydroxylic impurities. Impurities of this type can act as chain-stoppers in subsequent polymerization of the lactide to high molecular weight polymers required for biomedical and other uses. Therefore, it is desirable to keep the water and lactic acid contents of the lactide product as low as practicable.
The prior art depolymerization processes can require relatively long reaction times at high temperatures. These conditions can often result in side reactions, leading, for example, to unwanted isomers, including meso lactide, as well as charring of the polymer and consequently difficult-to-handle reactor heels.